The present invention relates to the production of synthesis gas and power by primary and secondary steam reforming of a hydrocarbon feedstock, wherein part of the synthesis gas being withdrawn from the secondary steam reforming at high pressure is expanded in a gas turbine for the production of power and the expanded synthesis gas is utilized as fuel in the primary steam reforming step.
Preparation of synthesis gas by a sequence of primary and secondary steam reforming of a hydrocarbon feed stock is a well-known process in the art. The process is conventionally performed in a sequence of a fired tubular steam reformer and an adiabatic combustion reactor with a top combustion zone and a bottom catalyst zone. In the combustion zone, hydrocarbon feedstock is partially oxidized with air or oxygen containing atmosphere. Partially oxidized effluent from the combustion zone is subsequently subjected to steam reforming in presence of a steam reforming catalyst arranged in fixed bed manner in the bottom part of the reactor. Typical operation conditions in the adiabatic reformer are 850.degree. C. and 2-4 MPa at steam to carbon ratios above 1, depending on the desired product gas.
A problem with adiabatic steam reforming is the formation of soot in the substoichiometric combustion of hydrocarbons. In particular, at low steam/carbon ratios in the feed gas to the adiabatic reformer, soot is formed markedly in the combustion zone.
In a number of industrial applications, a low steam to carbon ratio is required in the feed gas. Thus, in the manufacture of hydrogen and carbon monoxide, synthesis gas steam to carbon ratios below 1 are advantageous to obtain the optimum hydrogen to carbon monoxide ratio in the product gas.
Several attempts to reduce soot formation at low steam to carbon ratios in adiabatic reforming have been made in the past, including specific burner designs and control of operation conditions.
In co-pending European Patent Application No. 99102386, a process for soot free adiabatic catalytic steam reforming is described, wherein formation of soot is avoided by controlling the operation pressure within a certain range depending on the adiabatic gas temperature of the reformed feedstock and the steam to carbon ratio.
It was, furthermore, found that operation pressures above 3.5 MPa allow soot free reforming at a very low steam to carbon ratio.
A disadvantage of operating at high operation pressure in the adiabatic steam reforming process is the expense involved in compression of the feed gas. The pressure of produced synthesis gas will be typically required at a lower pressure for application in subsequent process units.